MOSCOW (Reuters) - A six-month-old boy became the seventh member of an Armenian family on Monday to die after a killing spree blamed on a Russian soldier that has strained ties between Moscow and Yerevan.

Armenian law enforcement officials say the soldier, Valery Permyakov, who is serving at a Russian military base in the tiny Caucasus nation, is their main suspect after military uniform boots with his name on them were found at the site where six members of the Avetisyan family were killed last week.

The baby, Sergei, died in hospital of his wounds.

The soldier’s motive remains unclear. Several thousand people staged protests last Thursday in the capital Yerevan and in Gyumri, Armenia’s second largest city, where the shootings took place, demanding the handover of Permyakov.

The incident has whipped up tension between Russia and Armenia, a former Soviet republic that normally enjoys close ties with Moscow and has signed up to a Russian-led Customs Union, a pet project of President Vladimir Putin.

In a telephone call on Sunday with Armenian President Serzh Sarksyan, Putin promised a swift investigation to bring the culprits to justice.

Russia’s federal Investigative Committee said its head, Alexander Bastrykin, had arrived in Yerevan on Monday to oversee the investigation, in which the conscript Permyakov is accused of murder and desertion.

It said that under Russian law, Permyakov could face a maximum punishment of life in prison if convicted.

In 1999, a court in Gyumri sentenced two soldiers from the same Russian base to 14 and 15 years in jail for killing two people and wounding several more in indiscriminate firing in the city, local media reported at the time.